Passive immunization with antibodies (Abs) is a recognized method of prophylaxis and treatment of infectious diseases. This approach may involve preparing human immunoglobulins from donors who recovered from an infectious disease and utilizing such preparations, containing Abs specific for the infectious organism, to protect a recipient against the same disease. Alternatively, therapeutic antibodies can be made by immunizing mice with an antigen, and then engineering/humanizing the mouse Ab into a human version. Monoclonal antibodies (mAbs) are homogeneous in terms of physical characteristics and immunochemical reactivity, and so offer the possibility of absolute specific activity.
That specificity can ultimately be a limitation for some targets, so practitioners have developed “bispecific” mAbs composed of fragments of two different mAbs and which bind to two different types of antigen. This facilitates binding to antigens expressed only weakly, for example. Some bispecific mAbs can stimulate strong immune responses, limiting their clinical application. One recent approach to ameliorating this effect is “CrossMab” methodology, a bispecific antibody format that adopts a more native antibody-like structure.
The prospects for generating a highly potent bispecific or bivalent antibody against a pathogen, such as HIV, for clinical use involves many uncertainties. The low spike density and spike structure on HIV may impede bivalent binding of antibodies to HIV, for example, and the geometry and spatial relationship of cell surface anchoring are not well-characterized. Nor is it known whether sufficient epitope accessibility on the HIV envelope exists. CrossMab bispecific antibodies that are anchored to a host cell membrane offer the possibility of improved local antibody concentration, targeting of sequential and/or interdependent entry steps, and compensating for monovalent binding.
Further still, large-scale, commercial production of antibodies remains challenging. For example, the production of therapeutic antibodies often requires the use of very large cell cultures followed by extensive purification steps, under Good Manufacturing Practice conditions, thereby resulting in extremely high production costs. Other limitations such as poor insolubility, protein aggregation, and protein instability can also make manufacturing of antibodies less than optimal.
Accordingly, there remains a need for therapeutically effective HIV antibodies that can be easily produced at a commercial scale.